In presses and dies, it has been common to utilize die cylinders which communicate with a manifold and function as die springs. It has also been suggested that self contained gas springs be provided as shown in U.S. Pat. Nos. 4,742,997 and 4,741,518.
A major consideration in connection with a design of self-contained gas springs or die cylinders is that when charged with a gas such as nitrogen at high pressures on the order of 2000 psi, the normal stroke of the piston substantially increases the pressure and the construction must be such as to withstand such high pressures. A further and important factor is the nature of the seals. Heretofore, it has been common to utilize U-shaped cross section seals which are subject to wear and leakage.
Among the objectives of the present invention are to provide a self-contained gas spring or die cylinder which is functional and operates at relatively high pressures; which includes a novel arrangement of seal assemblies; which is dependable, reliable and can be readily manufactured; which requires a minimum of maintenance, and which can be readily maintained and repaired.
In accordance with the invention, a self-contained gas spring or die cylinder, includes a cylindrical housing having a closed end, an open end and a cylindrical bore. A cylindrical sleeve extends into the bore of the housing and has at least a portion thereof forming an internal cylindrical surface. A piston rod extends through an opening in the sleeve and has a piston thereon contacting the cylindrical surface. The piston rod is hollow and has a closed end in the portion thereof external of the cylindrical housing. The sleeve supports a first seal assembly adjacent the inner end of the rod and engaging the external surface of the rod, an intermediate slide ring on the sleeve and a secondary seal assembly on the sleeve positioned such that the slide ring is between the primary seal and the secondary seal. The primary seal assembly is of the pressure relieving type so that on the inward movement of the piston rod a portion of the pressure within the housing passes to an intermediate area between the primary seal assembly and the secondary seal assembly such that the secondary seal assembly shares the sealing action with the primary seal. In a modified form, a third primary seal is provided on the sleeve between the sleeve and the cylindrical housing.